Investments - Schedule of Amortized Costs and Fair Values of Securities Available for Sale and Gross Unrealized Losses and Fair Values of Available-for-Sale Investments, Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Debt securities:
Total before allowance for credit losses	$ 8,438,760	$ 8,604,250
Gross Unrealized Gains	3,082	482,109
Gross Unrealized Losses	(830,209)	(17,413)
Fair Value	7,611,633	9,068,946
Allowance for Credit Losses	0	0	$ 0
Fair Value
Less than 12 Months	6,673,701	816,935
12 Months or Greater	653,807	27,109
Total	7,327,508	844,044
Unrealized Losses
Less than 12 Months	(635,555)	(15,844)
12 Months or Greater	(194,656)	(1,569)
Total	(830,211)	(17,413)
U.S. Government agencies
Debt securities:
Total before allowance for credit losses	21,003	43,472
Gross Unrealized Gains	0	1,071
Gross Unrealized Losses	(377)	0
Fair Value	20,626	44,543
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	20,626
12 Months or Greater	0
Total	20,626
Unrealized Losses
Less than 12 Months	(377)
12 Months or Greater	0
Total	(377)
U.S. Treasury
Debt securities:
Total before allowance for credit losses	2,813	2,469
Gross Unrealized Gains	6	21
Gross Unrealized Losses	(90)	0
Fair Value	2,729	2,490
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	1,749
12 Months or Greater	0
Total	1,749
Unrealized Losses
Less than 12 Months	(90)
12 Months or Greater	0
Total	(90)
States and political subdivisions
Debt securities:
Total before allowance for credit losses	476,338	479,148
Gross Unrealized Gains	668	27,733
Gross Unrealized Losses	(65,507)	(921)
Fair Value	411,499	505,960
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	346,009	38,853
12 Months or Greater	28,420	1,790
Total	374,429	40,643
Unrealized Losses
Less than 12 Months	(55,014)	(779)
12 Months or Greater	(10,493)	(142)
Total	(65,507)	(921)
Foreign governments
Debt securities:
Total before allowance for credit losses	62,964	62,979
Gross Unrealized Gains	0	293
Gross Unrealized Losses	(17,076)	(881)
Fair Value	45,888	62,391
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	22,591	31,862
12 Months or Greater	23,296	0
Total	45,887	31,862
Unrealized Losses
Less than 12 Months	(7,658)	(881)
12 Months or Greater	(9,420)	0
Total	(17,078)	(881)
Public utilities
Debt securities:
Total before allowance for credit losses	681,785	745,359
Gross Unrealized Gains	117	39,919
Gross Unrealized Losses	(66,765)	(309)
Fair Value	615,137	784,969
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	601,824	15,286
12 Months or Greater	10,747	0
Total	612,571	15,286
Unrealized Losses
Less than 12 Months	(61,970)	(309)
12 Months or Greater	(4,795)	0
Total	(66,765)	(309)
Corporate bonds
Debt securities:
Total before allowance for credit losses	6,199,886	6,322,471
Gross Unrealized Gains	1,940	391,287
Gross Unrealized Losses	(570,255)	(12,805)
Fair Value	5,631,571	6,700,953
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	4,985,075	541,974
12 Months or Greater	434,625	25,319
Total	5,419,700	567,293
Unrealized Losses
Less than 12 Months	(432,492)	(11,378)
12 Months or Greater	(137,763)	(1,427)
Total	(570,255)	(12,805)
Commercial mortgage-backed
Debt securities:
Total before allowance for credit losses	21,965	27,016
Gross Unrealized Gains	0	741
Gross Unrealized Losses	(1,680)	0
Fair Value	20,285	27,757
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	20,285
12 Months or Greater	0
Total	20,285
Unrealized Losses
Less than 12 Months	(1,680)
12 Months or Greater	0
Total	(1,680)
Residential mortgage-backed
Debt securities:
Total before allowance for credit losses	337,186	530,702
Gross Unrealized Gains	183	18,921
Gross Unrealized Losses	(16,338)	0
Fair Value	321,031	549,623
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	307,410
12 Months or Greater	0
Total	307,410
Unrealized Losses
Less than 12 Months	(16,338)
12 Months or Greater	0
Total	(16,338)
Asset-backed
Debt securities:
Total before allowance for credit losses	634,820	390,634
Gross Unrealized Gains	168	2,123
Gross Unrealized Losses	(92,121)	(2,497)
Fair Value	542,867	390,260
Allowance for Credit Losses	0
Fair Value
Less than 12 Months	368,132
12 Months or Greater	156,719
Total	524,851
Unrealized Losses
Less than 12 Months	(59,936)
12 Months or Greater	(32,185)
Total	$ (92,121)
Home equity
Fair Value
Less than 12 Months		188,960
12 Months or Greater		0
Total		188,960
Unrealized Losses
Less than 12 Months		(2,497)
12 Months or Greater		0
Total		$ (2,497)